Numerous products are packaged in films. Processing on packaging machines frequently necessitates films which are sealable. Great importance is attached to the sealing layer, since it predominantly determines the processing conditions on packaging machines. The strength, the commencement of sealing and the hot tack properties of the film layer are of particular importance here. Not all the requirements imposed on a packaging material can normally be fulfilled by one polymer. Polymers of different kinds are therefore combined. The use of similar polymers is desirable in order to improve the re-use of the film material.
Many films contain sealing layers based on polyethylene or copolymers thereof. Sealing is generally effected by one or two heated sealing jaws, which press the areas to be sealed together for a defmed period. The advantage of a polyethylene sealing layer, amongst others, is its low sealing temperature compared with other polymers.
A description and an analysis of the operations during sealing have been given by MEKA/STEHLING (Heat Sealing of Semicrystalline Polymer Films I; J. Appl. Polymer Sci., Vol. 51 (1994), pages 89-103). The sealing curve is described by these authors, amongst other features, by the sealing initiation temperature T.sub.Si, the temperature at the start of the sealing plateau T.sub.pi, and the sealing strength in the region of the sealing plateau SS.sub.p. In a further article (Heat Sealing of Semicrystalline Polymer Films II; J. Appl. Polymer Sci., Vol. 51 (1994), pages 105-119), STEHLING/MEKA discuss different polymers as sealing layers. They deal in particular with the physical properties of the polymers and their effect on sealing behavior. Thus STEHLING/MEKA state that there is a correlation between the yield point of the film samples obtained and their sealing strength. It is therefore to be expected that sealing layers with high yield points result in high sealing strengths and sealing layers with low yield points result in low sealing strengths. STEHLING/MEKA also state that the sealing initiation temperature for polyethylene can be reduced by the addition of comonomers. However, the yield point, and thus the sealing strength SS.sub.p also, is reduced by the introduction of a comonomer.
According to GNAUCK/FRUNDT (Einstieg in die Kunststoffchemie Introduction to the Chemistry of Plastics!, Carl Hanser Verlag, Munich, 3rd Edition, 1991, pages 59 et seq.), conventional ethylene copolymers rarely contain long-chain branches, and are therefore classed as linear polymers. Their mechanical and thermal properties vary approximately linearly as a function of their density and crystallinity. The properties and uses of these olefin copolymers substantially correspond to those of ethylene homopolymers.
Polyethylene, and copolymers which consist of ethylene and .alpha.-olefine, have quite recently been polymerized in the presence of highly stereospecific catalysts. Catalysts such as these are to be found in the group comprising metallocenes, for example. Metallocene catalysts comprise a catalyst system which consists of a soluble metal complex with a defmed structure which is fixed on a support material, and the activator (alumoxan). A fundamental definition of this class of substances is given by BEYER/WALTER (Lehrbuch der organischen Chemie Textbook of Organic Chemistry!; Hirzel Verlag Stuttgart 1991, 22nd Edition, page 65 et seq.). A survey of the use of metallocene catalysts in the polymerization of polyolefins is given by MUHLHAUPT (Nachr. Chem. Tech. Lab 41 (1993) No. 12, pages 1341-1351). Amongst other topics, the effect of different metallocene catalysts on the polymerization process is described, using polypropylene as an example. Polymers produced with conventional Natta-Ziegler catalysts frequently contain higher concentrations of .alpha.-olefines in the low molecular weight fractions. In a further publication (Die angewandte Makromolekulare Chemie Applied Macromolecular Chemistry!227, (1995), No. 3981, pages 159-177) HUNGENBERG ET AL. deal with the synthesis of olefin oligomers and polymers, and with their molecular weights and melting points.
A film consisting of linear polyethylene of low density is described in WO 94/14855. This film consists of ethylene which contains a copolymer and of an .alpha.-olefin with 3 to 10 carbon atoms. The density varies from 0.9 to 0.929 g/cm.sup.3 ; the melt flow index measured at 190.degree. C. according to ASTM D-1238, Condition F, varies from 15 to 25 g/10 min; the molecular weight distribution M.sub.w /M.sub.n varies from 2.5 to 3.0, and the crystallite melting point varies from 95.degree. to 135.degree. C. The opacity of the film according to ASTM D-1003 is between 3 and 20. The sealing properties of the film are not mentioned. The catalyst used contains a support material, aluminium oxan, and at least one metallocene.
EP 0572034 describes an ethylenic copolymer and an ethylenic copolymer composition. The melt index MFR is between 0.1 and 30 g/10 min, the density is between 0.88 and 0.94 g/cm.sup.3. The catalyst used for the production of the polymers contains metallocenes. The sealing temperatures of flat film specimens are mentioned. Sealing seam strengths and hot tack measurements are not mentioned.
WO 94/26816 describes a copolymer consisting of ethylene and a C.sub.4 to C.sub.12 comonomer with a narrow molecular weight distribution, and describes the production and use thereof, particularly as a stretch film. The catalyst used for the production of the polymers contains metallocenes. Sealing properties and hot tack measurements are not mentioned.
EP 0598626 describes an ethylene/.alpha.-olefin copolymer. The catalyst used for the production of the polymers contains at least two specific metallocenes. The polymer composition is characterised by its good thermoforming capacity, its high transparency and its adhesion to polar materials. Sealing properties and hot tack measurements are not mentioned.
Selected ethylene copolymers and ionomers were compared by J.R. DE GARAVILLE (TAPPI Proceedings: 1993, Polymers, Lamination & Coating Conference, pages 525-538) with an ethylene/vinyl acetate copolymer and with some linear polyethylenes of different densities. A copolymer consisting of ethylene and an .alpha.-olefin with 4 carbon atoms, which was produced with metallocene catalysts, was also included in the investigation. The results of this study by DE GARAVILLE show that ionomers and acid-modified ethylene copolymers are the most suitable polymers for flexible packaging as regards sealing properties and hot tack performance. The ethylene copolymer produced with metallocene catalysts was distinguished from conventional linear polyethylenes by a broader sealing range coupled with lower sealing strengths. However, a sealable film is desirable which is based on polyethylene and which is compatible with polyethylene. This is not obtained in all cases with ionomers. The object therefore arises of providing a sealable film which corresponds to the following requirements:
1. film consisting of polyethylene or copolymers thereof: PA1 2. low sealing temperature: PA1 3. high sealing strength: PA1 4. high sealing strength directly after sealing: PA1 5. optical requirements: PA1 polymerized with metallocene catalysts, PA1 crystallite melting point less than 110.degree. C., preferably less than 105.degree. C., PA1 melt index MPR from 0.5 to 10 g/10 min, PA1 molecular weight distribution M.sub.w /M.sub.n less than 3, preferably less than 2.5.
a polyethylene film is wanted, since polyethylenes are widely used standard plastics. The copolymers used should also predominantly consist of olefins and should be compatible with polyethylene. The concept of the standardization of material is thus addressed.
low sealing temperatures are generally desired, to ensure that packaging materials and filling materials are treated gently. An assessment corresponding to this requirement profile can be made by means of the sealing initiation temperature T.sub.si. PA2 The sealing layer should not only seal at low temperatures but should also exhibit high sealing strengths. The height of the sealing plateau is a measure of the sealing strength; the higher the plateau, the higher the mechanical loading to which the sealing seam can be subjected. PA2 A high sealing strength when the sealing seam is still warm is required for a multiplicity of packaging solutions. An examination of the hot tack is a suitable assessment criterion. PA2 The packaging must not adversely affect the presentation of the contents. A transparent film is generally required. The contents must be visible through the film as true to nature as possible. A suitable assessment criterion for this purpose is the absorption of visible light (haze).
The following definitions are valid within the scope of the invention:
All the cited polymers are commercially available products. For mixtures, the concentration of the individual polymers are given in % by weight unless indicated otherwise. The quoted densities are determined according to ISO 1183 at 23.degree. C. The melt flow index MFR is measured according to ISO 1133 at a temperature of 190.degree. C. and using a bearing weight of 2.16 kg, unless indicated otherwise. The polymers are abbreviated according to the agreed convention. Different polymers of the same class are distinguished by a hyphen and a number (example: LLDPE-2).
The outer layer of the film with the lowest melting point is designated as the "sealing layer". If the outer layers contain polymer mixtures, the outer face of the film which contains the lowest melting component is designated as the sealing layer. The sealing layer is written on the right when quoting film structures, unless indicated otherwise.
Polyethylene of low density which falls within the density range from 0.86 to 0.93 g/cm.sup.3 is designated as "LDPE". LDPE molecules are characterised by a high degree of branching.
Linear polyethylenes of low density, which in addition to ethylene contain one or more .alpha.-olefins with more than 3 C atoms as comonomers, are designated as "LLDPE". Butene-1, hexene-1, 4-methylpentene-1 and octene-1 can be cited as representatives of .alpha.-olefins here. The polymerization of the said substances results in the molecular structure which is typical of LLDPE, and which is characterised by a linear main chain with side chains suspended thereon. The density varies between 0.86 and 0.935 g/cm.sup.3. The melt flow index MFR is usually between 0.3 and 8 g/10 min. In some publications, linear ethylene/.alpha.-olefin copolymers are subdivided into VLDPEs or ULDPEs according to their density. However, since according to GNAUCK/FRUNDT (Einstieg in die Kunststoffchemie, Hanser Verlag 1991, page 58) the properties, processing and use of these copolymers substantially correspond to those of ethylene homopolymers, a more precise distinction is dispensed with here.
The term "MPE" here designates an ethylene copolymer which was polymerized by means of metallocene catalysts. An .alpha.-olefin with four or more carbon atoms is preferably used as the comonomer. Polymers produced with conventional Natta-Ziegler catalysts frequently contain higher concentrations of .alpha.-olefins in the low molecular weight fractions. Metallocene centres, which have a very uniform catalytic action, result in narrow molecular weight distributions, and a very uniform incorporation of .alpha.-olefins is observed on fractionation, both in the high and in the low molecular weight fractions. The density is preferably less than 0.92 g/cm.sup.3. The molecular weight distribution M.sub.w /M.sub.n is less than 3, preferably less than 2.5.
Copolymers consisting of ethylene and acrylic acid are designated as "EAA", and copolymers consisting of ethylene and methacrylic acid are designated as "EMAA". The ethylene content is preferably between 60 and 99 mole %.
Copolymers consisting of ethylene and vinyl acetate are designated as "EVA". The ethylene content is preferably between 60 and 99 mole %.
The layer or layers situated between the outer faces of the film are designated as "intermediate layer", "bonding layer" or "middle layer". The layers of a film are separated by a "/" stroke. Polymer mixtures are characterised by a "+", the polymers concerned being combined in round brackets. The quantitatively predominant component is named first.
There follows an example of the specified method of writing film structures. The three-layer structure LDPE/LLDPE/(MPE+LLDPE) has LDPE and the mixture (MPE+LLDPE) as its outer layers and LLDPE as its intermediate layer. The sealing layer is the mixture (MPE+LLDPE), wherein the MPE component predominates in relation to the LLDPE component.